System and method for the measurement of the layer thickness of a multi-layer pipe

ABSTRACT

System for measuring layer thicknesses of a multi-layer pipe by measuring with a detector array ( 2 ) the attenuation of an X-ray transmitted though the pipe. According to the invention the detector array ( 2 ) comprises an array of detector elements D 1 , D 2 , D 3 , D 4  with a collimator for defining the field of radiation in front of each detector element. The collimator has a narrow diaphragm aperture setting the resolution when the position of the pipe walls is to be determined. The defined field of radiation has an extent sufficient to radiate the four detector elements D 1 , D 2 , D 3 , D 4  in parallel. In a suitable signal processing of the output signals from the detector elements D 1 , D 2 , D 3 , D 4 , eg by using the method of least squares, the thicknesses of the different layers may be fairly accurately determined.

[0001] The present invention relates to a device for measuring the wall thickness of a tubular part.

[0002] In general, in the manufacture of tubular pipes by conventional rolling operations in the steel industry, the wall thickness of the pipe must be measured with a high degree of accuracy. In order to increase productivity, it is essential to measure the wall thickness of the pipe on-line without stopping the flow of products. Furthermore, since the rolling operations generally involve a hot rolling step at very high temperatures, it is desirable that the wall thickness of the pipe by measured not only in a non-contact manner, but also at a distance as far as possible from the tubular part.

[0003] A conventional wall thickness measuring device is shown in FIG. 1 of the accompanying drawings, wherein γ-ray sources 1,2 and 3 emit radiation which is detected by radiation detection units or sensors 4, 5 and 6. The tray sources 1 and 2 and the sensors 4 and 5 are mounted on a stationary frame 7 and the Bray source 3 and sensor 6 are mounted on a movable frame 8. The tubular part 11, whose wall thickness is to be measured, is conveyed along a conveyor 9 in a direction transverse to the direction of the γ-rays.

[0004] In this operation, the relative positions of the gray sources and the sensors are important factors. The movable frame 8 should be positioned as indicated in FIG. 2 of the accompanying 20 drawings, so that a regular triangle EFG is formed by the beams with the vertices of this triangle lying on the circumference of a circle whose diameter is the mean value of the nominal outside and inside diameters of the tubular part 11 (hereinafter referred to as “a middle diameter”). The principle of measurement will not be described in the present application, since it is already disclosed in laid-open Japanese Patent Application No. 46406/1981 and is not essential for understanding of the present invention.

[0005] As it moves along the conveyor 9, the tubular part 11 is at all times subject to vibration in the direction of axes Z₁-Z₂ and Z₃-Z₄, as indicated in FIG. 2. Accordingly, even if a vibration preventing roller (not shown) is added to the conveyor rollers 9, it is extremely difficult to set the vertices of the regular triangle EFG precisely on the circumference of the aforesaid circle. Furthermore, additional means to compensate for this vibration, such as the above-mentioned vibration preventing roller, include technical and very costly problems. The conventional measuring device shown in FIGS. 1 and 2 thus suffers from the drawback that its measurement theoretically includes an error due to vibration which is refered to as a “misalignment error”. Accordingly, the utilization of a vibration preventing roller in conjunction with a conveying roller 9 to minimise the vibration of the tubular part 11 and thereby minimise the alignment error has not been widely practiced.

[0006] Another method of measuring the wall thickness of a steel pipe by means of radiation is disclosed in laid-open Japanese Patent Application No. 114263/1979. This method is based on the fact that radiation applied to a steel pipe from outside suffers maximum attenuation when it passes tangentially to the inner surface of the pipe and minimum attenuation when it passes tangentially to the outer surface of the pipe. The points of such maximum and minimum attentuation are detected, and the wall thickness of the pipe is determined from the distance between these points.

[0007] However, when a steel pipe having a wall thickness between 5 or 6 mm and 40 mm is measured by this method, even if the radiation source employs a radioactive material of 30 curies, it takes at least 20 ms to 1 second for the measurement to be performed because the amount of radiation from the radioactive source is generally fractured. Therefore, during this period, the steel pipe must be held at rest. Accordingly, such a method cannot be used in measuring on-line the wall thickness of steel pipe which is vibrated while being conveyed. Furthermore, it can be understood that where the image of a radiation projected steel pipe is taken with a television camera, using a slit for projecting radiation from the radiation source having a width set at about 2 mm, the accuracy of the of the wall thickness measurement is much lower than that of a steel plate thickness gauge (several tens of micrometers) because the resolution of the television camera is only about 1 mm.

[0008] It is an object of the present invention to provide a device for measuring the wall thickness of a tubular part, which is capable of achieving a high degree of accuracy even if the tubular part undergoes vibration during the measurement.

[0009] Accordingly, such a device comprises a radiation source and a radiation detector disposed in spaced apart, aligned relation such that said tubular part can be received therebetween, the radiation source and the radiation detector each having a length which is greater than the outside diameter of the tubular part to be measured, so that parallel radiation beams emitted from the radiation source pass through the entire cross-section of the tubular part before reaching the detector, whereby the average wall thickness of said tubular part can be determined from the attenuation of the radiation detected by the detector.

[0010] The invention will now be further described, by way of example only, with reference to the remaining Figures of the accompanying drawings, in which:—

[0011]FIGS. 3a and 3 b are explanatory diagrams illustrating the basic measurement principle behind the present invention;

[0012]FIG. 4a is a sectional view of a radiation source and a collimator which form part of a measuring device according to the present invention;

[0013]FIG. 4b is a front view of the collimator shown in FIG. 4a;

[0014]FIG. 5a is a side view of a sensor and a collimator which also form part of the device of the invention;

[0015]FIG. 5b is a front view of the collimator shown in FIG. 5a;

[0016]FIG. 6 is a diagram for explaining the functional relationship between the wall thickness of a pipe and its position;

[0017]FIG. 7 is a graph indicating wall thicknesses with radiation detection elements of the sensor;

[0018]FIG. 8 is a schematic perspective view showing an example of a measuring device according to the present invention;

[0019]FIGS. 9a and 9 b are side and front views, respectively, of a two roll reducer; and

[0020]FIGS. 10a and 10 b are side and front views, respectively, of a three roll reducer.

[0021] The measurement principle behind the present invention will firstly be explained with reference to FIGS. 3a and 3 b. As seen in FIG. 3a, an array of tray sources (henceforth referred to as a line radiation source 21) and an assembly of sensors arranged in a line (henceforth referred to as line sensor 22) are located on opposite sides of a tubular part 11 whose wall thickness is to be measured. The length I of the line radiation source 21 and the line sensor 22 is set to a value which is much larger than the outside diameter of the pipe 11. The amount of attenuation of rays from the source 21 is measured by the sensor 22 so that an average wall thickness of the pipe 11 in the section can be obtained. As illustrated in FIG. 3b, the line sensor 22 detects the total radiation N_(o) when no pipe is present between the radiation source 21 and the sensor 22, and also the total count value N_(s) of the radiation when the pipe 11 is interposed between the source 21 and the sensor 22. The average wall thickness of the pipe 11 can be obtained from the two values N_(o) and N_(s). Because the length of the line radiation source 21 and the line sensor 22 is much larger than the outside diameter of the pipe 11, the total count values are not changed even when the pipe 11 is vibrated. Thus, according to the present invention, the average wall thickness of the pipe can be measured without any mis-alignment error.

[0022] The line radiation source 21 may be formed as shown in FIG. 4a. A radiation source holder 211 is set in a container having a recess 216. A plurality of radiation source capsules 212, for instance, a caesium 137, are arranged in a line within the holder 211. A rotary shutter 213 is arranged in the recess 216 of the container 210. The container 210 is coupled to a collimator 214 which has a number of collimator holes 215 arranged in a plurality of lines. The shutter 213 is turned by a rotating mechanism (not shown) in such a manner that a plate 217 of the shutter is held parallel to the plane of the Figure to allow radiation from the capsules 212 to pass to the collimator 214 during a measurement, and is held perpendicular to the plane of the Figure to block out the radiation when a measurement is not being carried out. Although the radiation is emitted radially from the capsules 212, it is converted by means of the holes 215 of the collimator 214 into parallel beams.

[0023] The line sensor 22 is shown in detail in FIGS. 5a and 5 b. A collimator 220 has a rectangular collimator hole 224 and a plastics scintillator 221 of polyvinyl toluene which is provided behind the collimator 220. A light guide 222 of acrylic is connected to the scintillator 221. A photo-multiplier tube 223 is coupled to the light guide 222 and the output of the tube 223 is applied to an amplifier (not shown). Radiation from the line radiation source 21 is formed into parallel beams by the holes 215 in the collimator 214 as described above. The parallel beams, after passing through the pipe 11, enter the hole 224 in the collimator 220. (Although only a single rectangular hole 224 is shown, it may be replaced by a number of collimator holes which are arranged in lines similar to the collimator holes 215 in the radiation source 221).

[0024] It is well known that the following fundamental equation is established for a radiation transmission-type thickness gauge.

N=N _(o) EXP(−μt)

[0025] where N is the detection output of the sensor when the radition passes through an object having a thickness t, N_(o) is the detection output or reference output of the sensor when no object is provided (i.e. when the thickness t equals 0), and μ is a constant or absorption coefficient.

[0026] As shown in FIG. 6, x- and y-axes can be defined relative to the pipe section. The wall thickness t_(i) of the pipe 11 at any given position x_(i) of the x-axis can be expressed as a function of that position as follows:

t _(i) =f(x _(i))

[0027] Therefore, the detection output N_(s) of the line sensor 22 as shown in FIG. 3a can be expressed in the following manner:

N _(s) = _(o)∫_(λ) N EXP(−μf(x))dx

[0028]FIG. 7 is a graph of In(N_(s)/N_(o)) against x, and indicates the wall, thickness t at various values of x together with the detection outputs N_(s) therefor. As is apparent from this Figure, a change in the value of In(N_(s)/N_(o)) by half represents a change S in the wall thickness (hereinafter referred to as “a half value layer”) of about 4.5 mm.

[0029] In general, if the half value layer is excessively large or small, the measurement becomes difficult. A half value layer for an ordinary flat plate which is measured by a transmission-type thickness gauge, which is extensively employed is about 11 mm. As the aforementioned value 4.5 mm is about half of this, when a measuring device employing the above-described principle is used, its measurement accuracy can be expected to be substantially the same as that of the above-mentioned gauge.

[0030] As is apparent from FIG. 7, the attenuation characteristic is linear in the wall thickness range from 3 mm to 15 mm (about 0.03 to 0.1 in the ratio t/d of wall thickness to diameter) and correction, which is required when the curve is non-linear, is unnecessary: that is, the average wall thickness of a tubular part can be determined directly from the detection output of the line sensor.

[0031] Using the measurement principle described above, a measuring device as shown in FIG. 8 can be used to determine the wall thickness of a tubular part or pipe 11. Rollers 9 are provided to convey the pipe 11 transversely of the measuring device. The length of a line radiation source 21 and a line sensor 22 (as described above) is set to a value much higher than the outside diameter D of the pipe 11. Such a measuring device has a high speed response and can measure the wall thickness of the pipe in a non-contact manner while the pipe is on-line, irrespective of any vibration caused as the pipe is being conveyed. The wall thickness thus measured can be fed back to workpiece rolling means to facilitate the control of speed or temperature therein thereby contributing to the quality control of the pipe.

[0032] Using the measurement principle described above, the half value layer is about one-half that of a flat plate, as previously described. This means that the variation in the quantity of radiation which is caused when a flat plate changes by 1 mm in thickness is equal to that in the quantity of radiation which is caused when a tubular part changes by about 0.5 mm in thickness. Accordingly the wall thickness of a pipe can be measured at least as finely as the thickness of a flat plate, so that the degree of accuracy is very high.

[0033] The measuring device according to the present invention is especially suitable for use with a stretch reducer. A stretch reducer is a mill which is used in the finishing rolling operation and is used for almost all small diameter seam pipes. It is also used for small diameter welded pipes because of its high degree of efficiency. In the stretch reducer, fourteen to twenty roll housings having two and three rolls are arranged successively along the pipe. While the outside diameter of the pipe is being rolled, the rolls of adjacent stands are rotated at different peripheral speeds, so that the pipe is pulled longitudinally-and the wall thickness is thereby controlled. Accordingly, if several kinds of pipes are provided then a variety of pipes different in diameter can be formed.

[0034] A two roll reducer is shown in FIGS. 9a and 9 b, while a three roll reducer is shown in FIGS. 10a and 10 b. In these figures the rolls 31 are shown in conjunction with a pipe 32 which is being pulled. With such a stretch reducer, the wall thickness of the pipe is changed by pulling it in the longitudinal direction. Therefore, in order to improve the control for operation of the rolling mill, it is essential to detect the average wall thickness of the pipe in the longitudinal direction rather than any irregularity in wall thickness in the pipe cross-section, especially in the case of a welded steel pipe which is manufactured from plate material of uniform thickness.

[0035] In the case where the speed of a multi-stage mill is changed to change the tension exerted on the pipe to control its wall thickness it is preferable that the response speed of the wall thickness measuring device is high. When the wall thickness measuring device is applied to the stretch reducer, it is generally located at the input or output side, i.e. at a point where the pipe is exposed to a considerable amount of vibration. However, there is no room for vibration-preventing pinch rollers in these locations. Accordingly, the measuring device of the present invention is particularly suited for use here because the measurements are not affected by the vibration of the pipe, and the response speed of the device is very high.

[0036] While the average wall thickness in the pipe section can be measured by conventional devices, such devices are extremely expensive since at least three radiation sources and three detectors are required. In contrast to this, the device of the present invention has only a single radiation source and a single detector, and therefore can be manufactured at much lower cost.

[0037] While the invention has been described with reference to a steel pipe, it should be noted that the technical concept of the present invention can be applied extensively to the measurement of the wall thickness of generally tubular pipes using gamma rays, X-rays, beta-rays, ultra-voilet rays, visible rays and infra-red rays, the radiation being chosen according to the material of the pipe, e.g. metal, plastics, glass, cement, etc. 

1. A device for measuring the wall thickness of a tubular part, comprising a radiation source and a radiation detector disposed in spaced apart, aligned relation such that said tubular part can be received therebetween, the radiation source and the radiation detector each having a length which is greater than the outside diameter of the tubular part to be measured, so that parallel radiation beams emitted from the radiation source pass through the entire cross-section of the tubular part before reaching the detector, whereby the, average wall thickness of said tubular part can be determined from the attenuation of the radiation detected by the detector.
 2. A measuring device as claimed in claim 1, wherein the radiation source and the radiation detector are each provided with a collimator for providing parallel radiation beams.
 3. A measuring device as claimed in claim 1 or 2, further comprising conveying means adapted to convey the tubular part transversely to said parallel radiation beams in a continuous on-line manner.
 4. A device for measuring the wall thickness of a tubular part, substantially as hereinbefore described with reference to FIGS. 3a to 5 b and FIG. 8 of the accompanying drawings. 